This invention relates to a pulse interleaver for return-to-zero optical pulses. In particular, it relates to a pulse interleaver for interleaving an input pulse train with itself, thereby providing an output with increased frequency. The invention also is concerned with variations in the state of polarization which can occur during the transmission of optical pulse trains through optical fibres.
Changes in polarization can adversely affect the performance of many different types of system. For example, polarization-induced signal fading is a recognised problem within optical communication systems. Polarization mode dispersion (PMD) also gives rise to polarization-dependent losses (PDL), and results from the birefringence of conventional optical fibres.
The conventional way to overcome polarization-induced signal fading is to use polarization maintaining fibres. However, these are costly and often difficult to incorporate effectively.
The use of Faraday rotator mirrors has been proposed as a replacement for reflectors at the ends of optical fibre arms in interferometer devices. A Faraday rotator mirror rotates a signal""s state of polarization by 45 degrees twicexe2x80x94once when the light enters, and once again when the light is reflected back into the fibre. Since the Faraday effect is non-reciprocal, the resultant state of polarization is rotated by 90 degrees with respect to the original signal. These rotations, applied in combination with a reversal of the polarization handedness upon reflection at the mirror, yields a state of polarization that is perpendicular to the original signal. It is thus possible to ensure that any state of polarization fluctuations that occur along the fibre anywhere between the source and the Faraday rotator mirror are exactly compensated and their unwanted effects neutralised. For example, Faraday rotator mirrors have been proposed to replace the conventional mirrors in a Michelson interferometer.
According to the present invention there is provided a pulse interleaver comprising:
an input for receiving a return-to-zero pulse train;
a beam splitter for dividing the input pulse train into at least two optical branches, a first one of the branches carrying a first signal, and the first one of the branches being terminated with a mirror and a device for manipulating the state of polarization, the reflected signal being re-combined at a combiner with a second signal on a second one of the branches,
wherein the effective path lengths for the first and second signals between the input and the combiner are selected such that the input pulse train is interleaved with itself thereby providing an output with increased frequency, and wherein the mirror and the device for non-reciprocal 45 degree rotation of the state of polarization provide a predetermined relationship between the polarization of pulses in the output pulse train derived from an individual pulse of the input pulse train.
The interleaver of the invention provides two path lengths for an input optical pulse train so that the pulse train is interleaved with itself thereby increasing the frequency. Such a device enables a pulse train to be generated using electro-optic circuitry with a first pulse frequency, and to increase the frequency purely in the optical domain. This places less demand on the frequency characteristics of the electro-optic circuitry.
In its simplest form, the invention enables the frequency of a pulse train to be doubled, by using a beam splitter which divides the input pulse train into exactly two optical branches for subsequent recombination. However, it is equally possible to divide the input pulse train into more than two branches to enable a greater increase in frequency.
The use of a device for manipulating the state of polarization enables compensation of any polarization mode dispersion in the optical paths within the interleaver. Thus, differential polarization mode dispersion does not affect interleaving of the pulses. The mirror and the device may together comprise a Faraday rotator mirror. Alternatively the device may comprise a 45 degree aligned quarter wave plate, where the PMD axes are defined or known, for example in the case of a planar waveguide implantation of the invention where PMD is caused by the asymmetry of the waveguide properties.
Preferably, the second one of the branches is also terminated with a mirror and a device for manipulating the state of polarization, and wherein the beam splitter and the combiner are implemented as a single optical coupler, the reflected signals from the first and second branches each returning to the optical coupler.
This arrangement effectively defines a similar configuration to a Michelson interferometer, although the path lengths between the optical coupler and the two mirrors are selected to interleave pulses, rather than analyse the superposition of those pulses. A phase control element may be provided in one branch, and an amplitude control element may be provided in the other branch to control the interleaving function. These two units may instead be provided in one of the branches. Furthermore, a modulator may be provided in each of the branches between the optical coupler and the respective mirror. This may enable an optical time division multiplexing system to be implemented.
Preferably, the predetermined relationship between the polarization of pulses may be that they have the same polarization. In this way, the output of the device is polarization-independent, so that all pulses derived from an individual pulse of the input pulse train have the same polarization as that input pulse train. The use of a polarization-independent interleaver makes the interleaver more suitable for use in certain systems, such as very high speed optical transmission systems.
An alternative configuration to the Michelson-type configuration described above is for the beam splitter to comprise a 3 dB 2xc3x972 coupler, the reflected signal from the first branch returning to the coupler and being at least partially transferred to a third branch, the signal on the third branch being re-combined with the second signal on the second one of the branches at the combiner, the combiner comprising a further coupler.
In this alternative arrangement, a single Faraday rotator mirror (or other arrangement) is used to provide the polarization control, although two separate couplers are then required, as opposed to the single coupler required in the Michelson-type arrangement.
In this case, an attenuator and a phase adjusting unit may be provided in the first branch between the 3 dB coupler and the mirror.
This arrangement enables the predetermined relationship between the polarization of pulses derived from an individual pulse of input train to be that they have orthogonal polarization. There are some applications where it is desirable to have adjacent pulses in the optical pulse train having orthogonal polarizations. For example when interleaving very high rate pulse streams, alternate orthogonally polarised pulses show lower pulse-pulse interaction because the adjacent pulse optical fields do not mix coherently.
An alternative design employs an optical circulator in the first one of the branches between the beam splitter and the mirror, the reflected signal being directed to a third branch by the circulator, the signal on the third branch being combined by the combiner with the second signal on the second one of the branches.
This further alternative design again enables alternate pulses to have orthogonal polarization. The use of a reflective spur (between the circulator and the mirror) enables orthogonal polarization of adjacent bits to be achieved more easily, and more stably.
The output pulses derived from an individual pulse of the input pulse train may be arranged to be adjacent each other. In such a case, the path difference is required to provide a differential delay of half the spacing between input pulses. Alternatively, the path lengths may be selected such that output pulses derived from an individual pulse of the input pulse train are spaced a predetermined number of pulses apart in the output pulse train, such that a signal sequence at the input is interleaved with itself with an interleaving depth dependent on the predetermined number.
This implementation enables a degree of randomness in the input pulse train to be preserved, by interleaving the pulse train with itself over a greater depth. This may be useful if the interleaver is used to increase the frequency of a test or quasi-random input signal.